This invention relates to an aqueous dispersion of an ionically crosslinked ethylene/.alpha.,.beta.-unsaturated carboxylic acid copolymer. More specifically, it relates to an aqueous dispersion having much improved stability and viscosity comprising a mixture of two ethylene/.alpha.,.beta.-unsaturated carboxylic acid copolymers or the metal salts thereof having different average contents of .alpha.,.beta.-unsaturated carboxylic acid units.
An ionically crosslinked ethylene/.alpha.,.beta.-unsaturated carboxylic acid copolymer is attracting attention as a coating material because of its excellent adhesion to various substrates. In coating, it is sometimes used in the form of an aqueous dispersion, and in this case, the aqueous dispersion must meet the following requirements.
(i) From the standpoint of coatability, the aqueous dispersion should have a moderate range of viscosity.
(ii) To obtain a necessary and sufficient coating thickness, the polymer concentration of the aqueous dispersion should be maintained above a certain point.
It is generally not easy to produce an aqueous dispersion of the ionically crosslinked ethylene/.alpha.,.beta.-unsaturated carboxylic acid copolymer which simultaneously meets these two requirements (i) and (ii). For example, attempts to obtain a stable aqueous dispersion having a high polymer concentration result in a very high viscosity that leads to poor coatability. Or if it is attempted to obtain an aqueous dispersion having a high copolymer concentration and a moderate viscosity, the stability of the resulting dispersion is poor and tends to separate into two layers. Because of these difficulties, no satisfactory aqueous dispersion of the ionically crosslinked ethylene/.alpha.,.beta.-unsaturated carboxylic acid copolymer has been discovered to date.
It has also been found that the particle diameters of the polymer particles and the distribution of the particle diameters vary with the content of the .alpha.,.beta.-unsaturated carboxylic acid unit in the aqueous dispersion of the ionically crosslinked ethylene/.alpha.,.beta.-unsaturated carboxylic acid copolymer, and the smoothness of the resulting coated film is extremely affected by the state of the particles, and that the water resistance of the coated film is directly changed by the content of the .alpha.,.beta.-unsaturated carboxylic acid unit.
Recently, Japanese Laid-Open Patent Publication No. 148534/77 suggested a coating aqueous dispersion of an ethylenic copolymer, said ethylenic copolymer being composed of 99 to 75 mole% of ethylene and 1 to 25 mole% of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, wherein the distribution of the .alpha.,.beta.-ethylenically unsaturated carboxylic acid group in the copolymer is characterized by the fact that the ratio of the reciprocal of the minus primary moment of distribution to the plus primary moment of distribution is from 0.50 to 0.95, and wherein at least 10% of the .alpha.,.beta.-ethylenically unsaturated carboxylic acid is neutralized with a metal ion and/or an ammonium ion. Certainly, the film-formability of this aqueous dispersion is improved to some extent. But when a surfactant is not used, it is impossible to form an aqueous dispersion having superior stability.